1. Field of Invention
This invention generally relates to data processing and transmission systems and more specifically to the interconnection of a plurality of devices or units by means of a common bus.
2. Discussion of Prior Art
It is known that a plurality of units or devices of a data processing system or data communication system may be connected to a single interconnecting bus over which all data transfers take place. Typical of such a system is the one described by U.S. Pat. Nos. 3,710,324 and 3,815,099 of Cohen et al.
It is typical of these systems that one of the devices, such as a processing unit, remains primarily in control of the operations taking place on the bus. Other devices may temporarily gain control of the bus for the limited time necessary to complete their data transfers, but they may obtain this control only by requesting it from the primary controlling device and having that request granted.
The above arrangement suffers from several disadvantages. Data transfers between devices on the bus depend upon the transmitting device having its request to obtain control of the bus granted by the primary controlling device. Thus, if the primary controlling device is busy or otherwise unable to grant the request, transmissions of data between other devices on the bus are delayed.
Another disadvantage of this arrangement is that the throughput of the primary controlling device is limited by the amount of time this device must spend in controlling data transfer between other devices. The maximum data handling capacity of this type of arrangement is also limited by the overhead introduced by the necessary communication of control information between the primary controlling device and the other devices of the system. Furthermore, a malfunction in the primary controlling device can effectively bring all data transmissions along the bus to a halt, even if the other devices on the bus remain in operating condition.
A further characteristic of some prior art systems is a requirement for a set of control lines to and from each separate device on the bus or the primary controlling device or both. Minimally, this set of control lines comprises a set of device address lines common to all devices. Each device must include means for recognizing the signals upon these lines and taking actions responsive to the signals. Minimizing the number of control lines would decrease the amount of hardware in the system which consequently would reduce its cost and increase its reliability. Furthermore, reduction of the number of control functions would reduce the overhead time necessary for system control and make this time available for transferring data, thereby increasing the maximum throughput of the system.
It is an object of this invention, therefore, to provide a data transmission control system in which data may be transferred between a plurality of devices without the requirement for one of these devices to remain primarily in control of the system.
It is another object of this invention to provide a data transmission control system for transmitting data between a plurality of devices in which the number of control lines interconnecting these devices is minimized.
It is yet another object of this invention to provide a data transmission control system for transmitting data between a plurality of computer-like devices in which the number of control functions which must be performed by each of these devices is minimized.